1. Field of the Invention
The present invention relates to a ceramic multilayer substrate and more particularly relates to a ceramic multilayer substrate in which surface electrodes are located on surfaces of a ceramic laminate including a plurality of ceramic layers.
2. Description of the Related Art
In general, a ceramic multilayer substrate has the structure in which surface electrodes and internal conductors are provided on surfaces of and inside a ceramic substrate (substrate body) containing a plurality of ceramic layers laminated to each other.
In the ceramic multilayer substrate as described above, in order to secure mounting reliability, the surface electrode is required to have a sufficient peeling strength so as not to cause any practical problems.
Incidentally, as one factor that decreases the peeling strength of the surface electrode provided on a surface of the ceramic multilayer substrate, for example, an influence of a plating liquid used in a plating step of forming a plating film on a surface of the surface electrode is known.
The reason for this is that since the plating liquid remains on the surface of the ceramic multilayer substrate, the interface between a ceramic and the surface electrode, both of which form the ceramic multilayer substrate, becomes fragile.
In addition, since the surface electrode is liable to be peeled off from its peripheral end portion (peripheral edge portion) which functions as a starting point, when the plating liquid remains at the peripheral end portion (peripheral edge portion) of the surface electrode, the peeling strength thereof is remarkably influenced.
For example, as shown in FIG. 18, when a main portion of a surface electrode 52 including a peripheral end portion (peripheral edge portion) thereof is formed on the surface of an outermost ceramic layer 51 which forms a ceramic multilayer substrate (see FIG. 11 of Japanese Unexamined Patent Application Publication No. 2004-104091), since a peripheral end portion (peripheral edge portion) 52a of the surface electrode 52 is located at a boundary with the ceramic layer 51, and a plating liquid is liable to remain at this position, the peeling strength of the surface electrode is liable to be decreased.
In addition, in the case (see FIG. 2 of Japanese Unexamined Patent Application Publication No. 2004-104091) in which the peripheral end portion (peripheral edge portion) 52a of the surface electrode 52 is covered with a cover ceramic layer 53 as shown in FIG. 19, the plating liquid may also enter through a boundary portion 53 between the ceramic layer 51 and the surface electrode 52 in some cases. In the case described above, the plating liquid reaches the peripheral end portion (peripheral edge portion) of the surface electrode 52 covered with the cover ceramic layer 53 and remains, and as a result, the peeling strength is disadvantageously decreased.
In addition, in the state in which the plating liquid remains, if a drying step is performed, the ceramic multilayer substrate is exposed to a drying temperature higher than ordinary temperature. In the case as described above, for example, when a Ni plating film (lower layer) and an Au plating film (upper layer) are each formed as a plating film, diffusion of Ni into an Au plating film occurs. The reason for this is that since the ionization tendency of Ni is higher than that of Au, Au is likely to be replaced with Ni. In addition, the diffusion of Ni may cause stains and discoloration, and as a result, appearance defects may occur in some cases. Furthermore, when a Ni plating film (lower layer) and a Sn plating film (upper layer) are each formed as the plating film, if the drying temperature is increased, oxidation and/or fusion of Sn may disadvantageously occur in some cases.